This invention relates to the control of combustion in a combustion system of the type comprising a combustion chamber, combustion means for causing the combustion of fuel in such chamber, a heat exchanger connecting with the combustion chamber for deriving useful heat from the combustion therein, an exhaust passage for removing the waste products of the combustion, fuel supply means for supplying fuel to the combustion means, and air supply means for supplying air to support the combustion.